LEDs are rapidly replacing incandescent bulbs, fluorescent bulbs, and other types of light sources due to their efficiency, small size, high reliability, and selectable color emission. A typical forward voltage drop for a high power LED is about 3-4 volts. The brightness of an LED is controlled by the current through the LED, which ranges from a few milliamps to an amp or more, depending on the type of LED. For this reason, LED drivers typically include some means to control the LED current.
In applications where high brightness is needed, multiple LEDs are used. It is common to connect LEDs in series, since the current through all the LEDs in series will be the same. The voltage needed to drive LEDs in series needs to be greater than the LEDs' combined forward voltages. For batteries or other power supplies that deliver 12 volts, only three or four LEDs can be connected in series. Therefore, boost converters are typically used in LED drivers that convert a low input voltage into a much higher voltage (e.g., up to 100 volts) to drive a selectable number of LEDs in series.
FIG. 1 is a typical prior art LED driver 10 that drives multiple LEDs 14 in series. Most components of the driver 10 are formed on an integrated circuit chip 12. Since the same chip 12 is used in one embodiment of the present invention, the operation of the driver 10 will be described in detail.
The driver 10 is a DC boost regulator that up-converts an input voltage (Vin) to the required output voltage (Vout) needed to drive the series-connected LEDs 14 at a desired regulated current. The regulator switches a switching transistor Q1 at a certain pulse-width modulation (PWM) duty cycle to maintain Vout at the required level. The switching is at a high frequency, such as 100 KHz-5 MHz, to keep component sizes small.
When the switching transistor Q1 is on, essentially connecting the inductor L1 between Vin and ground, a ramping current flows through the inductor L1, and the blocking diode D1 is off. Stored charge in the output capacitor Cout supplies a smooth current through the LEDs during this time. The blocking diode D1 prevents the capacitor Cout from discharging to ground when the transistor Q1 is on.
When transistor Q1 is turned off, the polarity of the voltage at the anode of diode D1 reverses, and diode D1 turns on. The stored inductor energy is then discharged, as a ramping down current, to recharge the capacitor Cout, while a smooth current flows through the LEDs. The relatively large value of the capacitor Cout maintains Vout at a relatively constant level (i.e., low ripple) to provide a smooth regulated current through the LEDs.
The duty cycle needed to maintain Vout (and thus the current) at the required level to drive the LEDs is set as follows. A low value resistor R1 in series with the LEDs has a voltage drop equal to ILED*R1. This voltage drop is a feedback voltage (Vfb) into the controller. An error amplifier 16 (an op amp) receives Vfb and a reference voltage (Vref) and generates an error signal related to the difference between Vfb and Vref. Any difference between Vfb and Vref causes the error signal to correspondingly charge or discharge a compensation capacitor Ccomp, through a compensation resistor Rcomp. The resulting voltage (Vcontrol) at the output of the amplifier 16 is relatively stable. The magnitude of Vcontrol is directly related to the duty cycle of the boost regulator, and the duty cycle is that required to cause Vfb to equal Vref (i.e., zero error signal).
A low value resistor R2 is connected in series with the switching transistor Q1 so that, when Q1 is on and conducting a ramping current through the inductor L1, the R2 voltage drop is a rising ramped voltage. This rising voltage is amplified, as required, by an amplifier 20 and applied to one input of a PWM comparator 22. The comparator's 22 other input is Vcontrol.
A clock is connected to the set input of an RS flip-flop 24 to set the Q output at the beginning of each clock cycle. The clock has a typical frequency between 100 KHz and 5 MHz. The high output of the flip-flop 24 at the start of the cycle is amplified by an amplifier 25, if necessary, to turn on the switching transistor Q1, shown as an N-channel MOSFET. The transistor can be any suitable type. The output of the comparator 22 is connected to the reset input of the flip-flop 24. When the rising voltage crosses Vcontrol, the output of the comparator 22 goes high and causes the Q output of the flip-flop 24 to be reset to zero to turn off the transistor Q1.
In this way, the duty cycle of the switching transistor Q1 is controlled to generate a smooth current through the LEDs required to cause Vfb to equal Vref. The value of resistor R1 can be selected to achieve any desired regulated current.
Numerous other types of boost regulators can also be used.
Common features in typical boost regulators used for driving LEDs in series are the blocking diode D1 and large, high voltage output capacitor Cout. The capacitor Cout must have a high voltage rating, such as 100 v, to handle the boosted voltage and any voltage spikes. Additionally, the value of the capacitor Cout is typically in the range of 1-10 μF so that there is only a small Vout ripple. When driving LEDs, a small variation in the driving voltage may cause a large variation in the current through the LEDs, making the brightness hard to accurately control. Such high value HV capacitors require a relatively large amount of space and are expensive.
Additionally, the blocking diode D1 is typically external to the controller IC chip 12 and must be purchased separately by the user and connected to the controller. Such an external diode and its connection add cost and uses space.
What is desired is a boost LED driver that is smaller and less expensive than the typically LED driver, such as shown in FIG. 1.